1. Field of the Invention
The present invention relates in general to the field of naphtha hydrotreating processes, and in particular, to systems and methods related to waste heat recovery for naphtha hydrotreating (NHT) processes for the desulfurization of naphtha.
2. Description of the Related Art
A petroleum refinery generally includes multiple separate unit operations and processes. One of the operations/processes includes the continuous distillation of petroleum to form a liquid distillate called naphtha which forms a major component of the refineries' product. After extraction through distillation, naphtha is generally further processed in catalytic reforming units, such as continuous catalyst circulation reactor (CCR) units, which require a certain naphtha feedstock specification to avoid degradation in the catalyst performance and reduced life of the downstream units. The naphtha and other distillates, however, in their initial form include numerous undesirable materials such as sulfur, nitrogen, olefins, and aromatics. In order to prevent damage to such units and to comply with new stringent environmental laws, worldwide, the distillates are subjected to a naphtha hydrotreating (NHT) process to remove the undesirable materials.
The naphtha hydrotreating process for removing such undesirable materials is one of the most mature process technologies in the oil refining processes and its use has become the norm in both old and new refineries. In the fifties of the last century, companies started to license naphtha hydrotreating processes under names such as “Unifining”; “Unionfining”, and other processes. Since then, the process has gone through many changes.
The naphtha hydrotreating process functions to remove the undesirable materials (e.g., sulfur, nitrogen, metals) from the petroleum distillates by selectively having these materials react with hydrogen in a catalyst bed at elevated temperature in a reactor unit. The chemistry behind the naphtha hydrotreating process can be divided into a number of reaction categories, called hydro-desulfurization, hydro-denitrification, saturation of olefins, and saturation of aromatics. For each of these reactions, hydrogen is normally used to achieve the desired quality of the petroleum fraction.
Desulfurization is by far the most common of the naphtha hydrotreating reactions. The content and form of the sulfur in hydrocarbons can vary. The reaction rates for different forms of sulfur can also vary significantly. For example, a Thiophenol reaction that results in benzene and hydrogen sulfide (H2S) is typically very rapid.
The degree at which sulfur can be removed from the hydrocarbons can vary from one petroleum distillate to another. In naphtha, however, sulfur removal can reach to near complete. Regardless of the content, form, or reaction rate, the desulfurization reaction results in the production of H2S in the reactor. To complete the desulfurization process, such H2S is removed in a downstream fractionation unit.
A main use of the naphtha hydrotreating process in naphtha applications is in the preparation of feed stocks provided to a naphtha reforming unit. A typical hydrotreating process includes a reactor section, a stripper section, and a naphtha splitter section. The reaction section provides hydrogeneration, desulfurization, denitrogenation reactions on a hydrotreatment catalyst. In the reaction section, hydrogen is combined with the feed and the stream is heated up to the desired hydrotreating temperature using a fired heater. The combined feed and hydrogen stream passes downward in a hydrogenation reactor packed with various types of catalyst depending upon reactions desired. The reactor effluent is cooled and a liquid phase from the cooled effluent is sent to the contaminants stripping section. This cooled effluent from the reaction section is preheated, usually against the stripper bottom product stream and then sent to the stripping column. The stripper effluent is cooled and enters the high pressure separator which separates the liquid hydrocarbon from the hydrogen/hydrogen sulfide/ammonia gas. The acid gases are absorbed from the hydrogen in an amine absorber, and hydrogen, minus purges, is recycled with make-up hydrogen. The stripper bottom product provides the “feed” to a naphtha splitter section. This “feed” is first preheated against the splitter bottom stream and sent to the splitter section. In the splitter section, light naphtha is separated from heavy naphtha, which is used as “feed” by a continuous catalyst regeneration and reforming unit (CCR) to produce high octane components. The naphtha hydrotreating process reduces the sulfur and nitrogen in the feedstock to the downstream catalytic reforming process unit to less than 0.5 wt ppm and the metals to non-detectable levels.
Most of the old and recently built naphtha hydrotreating plants use either Axens or UOP processes. These two conventional configurations are almost the same with respect to the configuration of the contaminants stripper and naphtha splitter sections. Recognized by the inventors is that neither configuration exhibits direct integration between the two sections.
Waste heat recovery has been employed in conventional naphtha hydrotreating processes, including in these exemplary processes, in order to reduce the amount of energy consumed. In such processes, the feed to the respect to section is typically preheated by the bottom product of the section and the extra waste heat in the heavy naphtha stream from the naphtha splitter section is sent to the air and water coolers.
Although very simple, the contaminants stripper and a naphtha splitter sections however, use huge amounts of heating utilities, most of the time in the form of fossil fuel consumed in re-firing units assigned to each of the sections. These fired heaters, used to supply the required heating utility, also produce a large quantity of undesirable emissions. In some parts of the world, such emissions are catastrophic to the environment. As noted above, the stripper and splitter sections also require huge cooling utilities in form of air coolers and water coolers. Air coolers are capital and maintenance intensive equipment and water cooling, and in some parts of the world have significant availability and maintenance problems, as well.
Recognized by the inventors is that, even in view of such difficulties surrounding the use of external utilities, such conventional energy recovery methods fail to optimize waste heat recovery within and between the processes. Accordingly, the inventors have recognized that it would be beneficial to the oil refining industry to hydrotreat different categories/types of naphtha feed stocks destined for a refining reforming unit and other applications with less energy consumption than conventionally possible, while producing less green house gas emissions, and/or using a lesser number of heaters and correspondingly less capital investment in such heaters, air coolers, and water coolers.
Particularly, recognized by the inventors is that it would be beneficial to the oil refining industry to hydrotreat different naphtha feed stocks destined for a refining reforming unit with less energy consumption by (destined for) fired heaters and with less energy consumption by air and/or water coolers, while producing less green house gas emissions. Further, recognized by the inventors is that it would be beneficial to the oil refining industry to hydrotreat the naphtha feed stocks destined for a refining reforming unit using a lesser number of heaters and using less capital investment in plant's heaters, air and water coolers. Still further, recognized by the inventors is that it would be beneficial to the oil refining industry to hydrotreat the naphtha feed stocks destined for refining reforming unit using a process configuration that can be used worldwide in any naphtha hydrotreating process including in places with extreme differences in energy cost. Additionally, it would be also very beneficial to have a naphtha hydrotreating process reaction furnace that is flexible and with a low beta ratio to handle different feed stocks. It would also be extremely beneficial to the refining industry to have a naphtha hydrotreating process configuration with an efficient waste heat recovery system that is retrofitable for more efficient energy usage along the lifetime of the naphtha hydrotreating plant.